The present invention relates to the field of surge arrestors.
It applies in particular to arrestors for medium voltage, typically to electricity networks in which the nominal root-mean-square voltage between phases is greater than 1 kilovolt (kV).
Surge arrestors are devices designed to be connected between ground and an electricity line, in particular at medium or high voltage, for the purpose of limiting the amplitude and the duration of voltage surges that appear on the line.
These surges can be due, for example, to atmospheric phenomena, such as lightning, or to induction phenomena in the conductors.
These surges can also be due to switching operations on a line that is live.
Surge arrestors are generally built up as a stack of varistors, and nowadays usually by a stack comprising a plurality of disks based on zinc oxide, whose electrical resistivity is highly non-linear as a function of applied voltage.
More precisely, such varistors pass practically no current so long as the voltage across their terminals is below a conduction threshold, and in contrast, they pass a very high current, that can be as much as several tens of kiloamps (kA) when the voltage applied across their terminals exceeds the above-mentioned conduction threshold.
The number of varistors used in a surge arrestor is such that the operating voltage of the electricity line is below the conduction threshold of the stack of varistors.
Thus, the arrestor can withstand the service voltage continuously and without significant current leakage, while nevertheless making it possible to pass very high levels of discharge current that can appear temporarily on a line in the event of an accidental surge. Numerous types of arrestors have already been proposed.
The arrestor field has given rise to very abundant literature.
At present, known arrestors generally comprise:                a stack of varistors;        two contact pieces of electrically conductive material placed at respective ends of the stack of varistors; and        an envelope of electrically insulating material surrounding the stack of varistors.        
The above-mentioned envelope of electrically insulating material is itself the subject of very abundant literature.
For example, document GB-A-2 073 965 proposes making the envelope out of a heat-shrink material.
Documents U.S. Pat. No. 4,298,900, DE-A-3 001 943, and DE-A-3 002 014 has proposed also placing an outer case of porcelain over the heat-shrink envelope.
Documents U.S. Pat. No. 4,092,694 and U.S. Pat. No. 4,100,588 have proposed placing each varistor in a silicone-based ring and placing the stack of varistors surrounded in this way in a case of porcelain.
Document U.S. Pat. No. 2,050,334 proposes placing a stack of varistors in a porcelain case and filling the gap between the case of porcelain and the stack of varistors with a filler material, e.g. formed by a halogenated wax-based compound.
Documents EP-A-0 008 181, EP-A-0 274 674, EP-A-0 231 245, and U.S. Pat. No. 4,456,942 propose making the envelope surrounding the varistors out of an elastomer material, formed in position by overmolding.
More precisely, EP-A-0 274 674 proposes overmolding an envelope out of composite material based on elastomer, ethylene-propylene diane monomer (EPDM), silicone, or some other optionally filled resin, on a stack of varistors.
Document U.S. Pat. No. 4,161,012 also proposes placing an envelope of elastomer on the varistors. That document proposes making the envelope by depositing the elastomer on the outside surface of the varistors, or by molding the envelope on the varistors, or indeed by preforming the elastomer envelope and then inserting the varistors therein.
As early as 1958, Document U.S. Pat. No. 3,018,406 proposed making the envelope in the form of two complementary pre-shaped shells together with an outer envelope of plastics material injection-molded onto the varistors.
Document U.S. Pat. No. 3,586,934 proposes making the envelope out of a synthetic resin, e.g. based on epoxy or polyester, or indeed out of polyester or silicone varnish.
Document EP-A-0 196 370 proposes making the envelope on a body of varistors by casting a synthetic resin, e.g. constituted by epoxy resin, polymer concrete, silicone resin, or elastomer, or by covering the body of varistors in a heat-shrink tube of plastics material, or indeed providing said stack with a layer of synthetic resin.
In addition, documents U.S. Pat. No. 4,656,555, U.S. Pat. No. 4,905,118, U.S. Pat. No. 4,404,614, EP-A-0 304 690, EP-A-0 335 479, EP-A-0 335 480, EP-A-0 397 163, EP-A-0 233 022, EP-A-0 443 286, and DE-A-0 898 603 propose making the envelope surrounding the stack of varistors out of composite materials made up of fibers, generally resin-impregnated glass fibers.
More precisely, document U.S. Pat. No. 4,656,555 proposes initially forming a winding of fibers based on plastics material such as polyethylene, or glass, or indeed ceramic, optionally impregnated in resin, e.g. epoxy, and then forming on the outside of the winding a case of weatherproof polymer material, e.g. a case based on elastomer polymers, synthetic rubber, thermoplastic elastomers, or EPDM.
More precisely, that document proposes either preforming the weatherproofing polymer case and then engaging the stack of varistors fitted with the fiber winding inside the case, or else initially forming the fiber winding on the stack of varistors and then making the case of weatherproofing polymer material by molding it onto the winding, by spraying polymer onto the winding, or by dipping the stack of varistors provided with the winding in a bath of polymer.
Document U.S. Pat. No. 4,404,614 proposes placing in succession on a stack of varistors: a first envelope based on resin-impregnated glass fibers, e.g. impregnated with epoxy resin; then a second envelope based on glass flakes and epoxy resin; and finally an elastic outer envelope based on EPDM rubber or on butyl rubber.
That document states that the first envelope, the second envelope, and the outer envelope can be put into place in succession the stack of varistors, or the envelopes can be made in the opposite order.
That document also mentions the possibility of molding the outer envelope on the second envelope based on glass flakes and epoxy resin.
Document EP-A-0 233 022 proposes forming on a stack of varistors a shell that is based on glass fibers reinforced by epoxy resin, and then an elastomer-based envelope that is heat-shrinkable, or that can be released by equivalent mechanical means onto said shell.
In a variant, the envelope can be molded in situ using a synthetic resin or a polymer material.
The document states that the shell can be preformed. The document also proposes using a sheet of preimpregnated fibers.
Document EP-A-0 304 690 proposes beginning with a filamentary winding of glass fibers impregnated in resin, and then forming a coating on the outside of the winding by injecting an EPDM type elastomer material.
Document EP-A-0 355 479 proposes placing in succession on the stack of varistors, firstly a barrier formed by a plastics film, e.g. based on propylene, then a winding of non-conductive filaments, and finally an elastomer case that is weatherproof.
Document EP-A-0 397 163 proposes placing in succession on the stack of varistors, a winding of resin-impregnated filaments, and then forming a coating of elastomer flakes on said winding, e.g. by injecting EPDM.
The technique of using a composite material is very old.
In 1964, document DE-A-0 898 603 was already proposing to use resin-impregnated glass fibers to envelop varistors.
More recently, document FR-A-2 698 736 has proposed a method of manufacturing an arrestor comprising the steps which consist in stacking varistors, forming a first envelope of composite material on the stack of varistors, which first envelope is at least semi-rigid and presents a constant external section along its length to serve in particular to compensate for surface irregularities of the stack of varistors due to misalignments and to dispersions in varistor dimensions, and then in placing an outer envelope having fins or “sheds” on the first envelope, the outer envelope being made of composite material of substantially constant thickness on the first envelope and then fitting annular fins on the extruded annular envelope.
Furthermore, document WO-A-97/39462 describes a method of manufacturing arrestors comprising the steps which consist in:                stacking varistors; and        forming an envelope of composite material on the stack of varistors;        
wherein the step of forming an envelope of composite material consists in:                placing a fiber fabric on the outside of the stack of varistors and in contact therewith;        placing a flexible outer envelope on the outside of the stack of varistors; and        injecting a material suitable for impregnating the fiber fabric into the annular space formed between the stack of varistors and the flexible outer envelope.        
Arrestors that have been proposed so far have provided good service.
Nevertheless, they do not always give full satisfaction.
In particular, the Applicant has observed that nearly all presently-manufactured arrestors are made on the basis of enameled varistors.
Until now it has been considered essential to enamel varistors in order to improve their dielectric strength and also to establish a chemical barrier against their immediate environment.
The Applicant has observed that this enameling gives rise to two problems.
Firstly, the enameling which generally contains a large content of lead (Pb), typically greater than 50%, leads to a major risk of polluting the environment, unless precautions are taken during manufacture for the recovery and/or recycling of used arrestors, which is inevitably expensive.
Secondly, enameling can be performed only prior to stacking, and consequently must be performed individually, varistor by varistor, giving rise to a non-negligible contribution to the overall cost price of present-day arrestors.
Starting from this observation, the Applicant proposes the present invention for improving existing arrestors.
A main object of the present invention is to make reliable arrestors based on varistors but without requiring enameling.